Two-stage air-hydraulic booster

ABSTRACT

A two-stage air-hydraulic booster having two pumping chambers, with one being a relatively large-volume, low-pressure chamber and the other a relatively small-volume, high-pressure chamber and with both chambers being formed, in part, by pumping structure carried by an air-operated piston. The ratio of the area of the air-operated piston to the area of the hydraulic cylinder for the large-volume, low-pressure chamber is relatively low whereby a large volume of fluid per inch of stroke is delivered to the system while the ratio of the area of the airoperated piston to the area of the high-pressure, small-volume chamber is high to deliver a small volume of fluid per inch of stroke of the air-operated piston. The structure provides for minimal consumption of operating air in both advancing the movable air-operated piston in a pumping stroke and in return of the air-operated piston to an initial position.

United States Patent [191 McClocklin et al.

[4 Jan. 22, 1974 TWO-STAGE AIR-HYDRAULIC BOOSTER [75] Inventors: Samuel B. McClocklin; Clarence'L.

Kostelecky, both of Owatonna, Minn.

[73] Assignee: Owatonna Tool Company,

Owatonna, Minn.

[22] Filed: Dec. 26, 1972 [21] Appl. No.: 318,088

[52] US. Cl 417/302, 417/393, 417/401, 60/547, 60/578 [51] Int. Cl. F04b 17/00, F04b 35/00, Fl5b 7/00 [58] Field of Search..... 60/547, 560, 574, 575, 576, 60/577, 578; 417/302, 62, 225, 392, 393,

[56] References Cited UNITED STATES PATENTS 3,625,006 10/1969 Schoenleben 60/578 10/1968 Beck 6/1904 Albee 60/578 Primary Examiner-Edgar W. Geoghegan Assistant Examiner-A. M. Zupcic [57] WW i m Attorney, Agent, or finmzk srsatfiasrtlliet Stellman & Mcffiid A two-stage air-hydraulic booster having two pumping chambers, with one being a relatively large-volume, low-pressure chamber and the other a relatively smallvolume, high-pressure chamber and with both chambers being formed, in part, by pumping structure car- I ried by an air-operated piston. The ratio of the area of the air-operated piston to the area of the hydraulic cylinder for the large-volume, low-pressure chamber is relatively low whereby a large volume of fluid per inch of stroke is delivered to the system while the ratio of i the area of the air-operated piston to the area of the high-pressure, small-volume chamber is high to deliver a small volume of fluid per inch of stroke of the airoperated piston. The structure provides for minimal consumption of operating air in both advancing the movable air-operated piston in a pumping stroke and in return of the air-operated piston to an initial position.

15 Claims, 3 Drawing Figures II III TWO-STAGE AIR-HYDRAULIC BOOSTER BACKGROUND OF THE INVENTION This invention pertains to a two-stage air-hydraulic booster wherein an air-operated piston has structure associated therewith which coacts with additional elements to form high and low pressure pumping chambers and pumping elements which deliver a relatively large volume of low-pressure hydraulic fluid and, when a certain pressure is reached, deliver a smaller volume of high-pressure hydraulic fluid.

The assignee of this application is the owner of Schoenleben US. Pat. No. 3,625,006 which discloses a twostage hydraulic boosterpln the structure of this Patent, the air-operated piston is mounted in an air cylinder with opposite planar faces of the piston being subject alternately to air pressure for advance and retraction of the air-operatedpiston and the pumping structure associated therewith. Hydraulic boosters of this type require almost as much air in returning the air-operated piston to initial position as required in advancing the air-operated piston.

. Boosters are shown in Bahniuk U.S. Pat. No. 2,749,845 and in a pending application owned by the assignee loftthis application, namely Schoenleben application, Ser. No. 154,891, filed June 21, 1971. These boosters do not minimize air consumption in return of the ainoperated piston.

SUMMARY I to provide a two-stage air-hydraulic booster with structure which minimizes air .eonsumption in providing the desired supply of hydraulic fluid under low and high pressure and in return of theair-operated piston back to its initial position.

In the structure disclosed herein, a two-stage airhydraulic booster is provided wherein an air-operated piston is movably mounted within a cylinder and has structure associated therewith which, in part, provides a low-pressure, high-volume pumping chamber and a high-pressure, low-volume pumping chamber wherein maximum hydraulic flow is obtained in apumping stroke and. with a substantially lesser amount of air being required to return the air piston to initial position.

More particularly, the air-operated piston has an annular member movable between the exterior surface of a centrally-positioned block and the inner wall of a hydraulic fluid reservoir and with suitable flow passages whereby the annular member and block define the high-volume, low-pressure chamber which reduces in size as the air-operated piston advances from its initial position. An exposed end of the annular member is subjected to pressure air on the return cycle of the airoperated piston whereby a minimum amount of air acting against the exposed end of the annular member returns the air-operated piston and associated structure to an initial position. This exposed end of the annular member has an area which is a small fraction of the area of the air-operated piston exposed to air in advance of the air-operated piston whereby a much smaller volume of air is required to return the airoperated piston. Thisprovides a faster return of tne airoperated piston than previously known and for faster operation of the structure operated by the hydraulic fluid.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of the two-stage airhydraulic booster with the booster assembly and control components shown in vertical central section and with the air-operated piston and parts movable therewith shown in initial position;

FIG. 2 is a view, similar to FIG. 1 with parts shown in position when the booster shifts from the first stage fast approach to the second stage high'pressure stroke; and

FIG. 3 is a view, similar to FIG. 1, showing the parts positioned for release of the hydraulic pressure and return of the air-operated piston and associated components to initial position.

DESCRIPTION OF THE PREFERRED EMBODIMENT The booster assembly has a pair of end plates and 11 secured together by a plurality of tie rods 12 and capturing thcrebetween a cylindrical member 15 having one end fitted into the end plate 11 and the other end fitted onto an end of an annular hydraulic fluid reservoir 16. The reservoir 16 is generally U-shaped in cross section and has a cylindrical outer wall 17 and an inner wall 18 with the upper ends of these walls fitted onto a shaped inner face of the end plate 10. The interior of the reservoir provides for storage of hydraulic fluid used by the booster. The cylindrical wall 15 in association with the end plate 11 and the end of the hydraulic fluid reservoir 16 defines an air chamber in which an air-operated piston 20 is movably positioned. One face 21 of the air-operated piston is selectively subject to air under pressure by air flow through a tubular member 22 extended between the end plates 10 and 11, with the upper end of the tube 22 communicating with a passage 23 in the end plate 10. The lower end of the tube 22 communicates with a passage 24 in the end plate 11 leading to a port in communication with the face 21 of the air-operated piston.

Means defining a large-volume, low-pressure hydraulic chamber includes an annular sleeve member 30 extending upwardly from a face 31 of the air piston 20 which surrounds and coacts with a centrally-positioned block 32 extending inwardly from the end plate [0. Suitable seals, as indicated at 33,34, and 35 are posi-' tioned between the reservoir 16, the block 32 and the inner surface of the end plate 10.

l The low pressure chamber is in flow communication with the exterior of the booster assembly body through a passage 36 in the block 32 which communicates with a passage 37 in the end plate 10. The annular member 30 moves within a chamber 40, defined by a space between the inner wall 18 of the reservoir 16 and the outer surface of the block 32, with the annular member 30 carrying a seal member 41 engaging the inner wall 18 of the reservoir and the block 32 carrying seals 42 and 43 engageable with the inner surface of the annular member 30. When pressure air is supplied to the tube 22, the air is supplied to the face 21 of the air-operated piston 20 to move the air-operated piston and annular member 30 upwardly to reduce the size of the lowpressure chamber and direct hydraulic fluid outwardly through the passage 36 in the block 32.

The small-volume, high-pressure chamber is defined by a chamber 45 in the block 32 with the air-operated piston 20 carrying a plunger or piston 46 movable within said chamber whereby upward movement of the air-operated piston 20, as shown in FIG. 1, causes upward movement of the piston 46 to reduce the size of the chamber 45 and force hydraulic fluid to flow outwardly of the booster assembly body through a passage 47 extending through the end plate 10.

The booster assembly body has a second air passage 48 through the end plate leading to the chamber 40 whereby pressure air may be directed selectively against the upper exposed end of the annular member 30 to move the air-operated piston downwardly to the initial position shown in FIG. 1.

The two-stage air-hydraulic booster is usable for supplying hydraulic fluid to one or more operating devices. A hydraulic clamp is shown at 50 as being typical of such an operating device. The control for supply of hydraulic fluid to the operating device 50 includes an air control circuit and a hydraulic control circuit. The air control circuit includes a pilot valve 51, an air control valve 52 and an air pressure regulator 53.

The hydraulic control circuit includes a hydraulic control valve body 54 having an air piloted release valve, indicated generally at 55, a check valve 56 and an air piloted unloading valve, indicated generally at 57. The control circuit also includes a pilot-operated check valve 58.

With the air-operated piston and associated parts in initial position, as shown in FIG. 1, the pilot valve 51 is positioned, as shown in H6. 1, to have a supply of air under pressure in a supply line 60 flow through a passage 61 to the upper end of a valve bore in the air control valve 52 and act against the upper end of a valve spool 62 to urge the valve spool downwardly to the position which is shown in H6. 1, against the action of a spring 62a. This places the air supply from supply line 60 in communication with the air pressure regulator 53 by connecting passage 60 with a passage 63 leading to the air pressure regulator and air then flows through a passage 64 into pilot chambers for the release valve 55 and the unloading valve 57. The pilot section for the release valve 55 includes a piston 65 having a stem 66 extending to a point adjacent a poppet valve member 67 urged to the right, as viewed in FIG. 1, against a seat by a spring 68. Air from the passage 64, by flow through a passage 69 in the valve body 54, acts against the left-hand face of the piston 65 to urge the pilot piston to the right and the pin 66 away from engagement with the release valve member 67. Pressure air also acts against a right-hand face of a pilot piston 71 of the unloading valve, as viewed in FIG. 1, to urge the pilot piston 71 to the left and against a poppet valve member 72 to urge the poppet valve member against its seat.

With the control valve member 62 positioned as shown in FIG. 1, the upper side of the operating device 50 is connected to an exhaust air line through a conduit 75 which extends through the check valve 58 and into the valve body 54. A passage in valve body 54 connects the conduit 75 to a conduit 76 extending to the body of the air control valve 52 with exhaust air then flowing through the body of valve 52 and through a passage 77 to the exhaust line 78. Air under pressure in passage 64, which acts upon the pilot 71 of the unloading valve, also communicates with the air tube 22 through a pair of passages 80,81 leading to the passage 23 in communication with the tube 22. interposed between passages 80 and 81 is a diaphragm-type quick exhaust valve 83 which, as shown in FIG. 1, permits communication between passages 80 and 81 and blocks communication to the exhaust line 78.

In operation, air under pressure directed to the face 21 of the air-operated piston 20 causes movement of the air-operated piston, the annular member 30, and the plunger 46. This causes flow of hydraulic fluid through the passage 36 and also from the chamber 45. These flows combine and reach a conduit communicating with a piston 91 in the operating device 50. The flow from the small-volume chamber 45 is through the passage 47 and through a passage 92 in the valve body 54 with the flow being around the check valve 56 and the release valve 55 and then through the check valve 58. The flow from the large-volume chamber passes through the passage 36 and to a passage 95 in the valve body 54 with the flow being around the stem of the unloading valve 72 and through the check valve 56 which is away from its seat due to the pressure of the hydraulic fluid acting thereagainst. The flow joins the flow from the chamber 45 downstream of the check valve 56.

During this operation, the chamber 40 is connected to exhaust by the passage 48 communicating with an air passage in the valve body 54 which connects with the passage 76.

In this operation, a large volume of low-pressure hydraulic fluid is delivered to one or more operating devices 50 with the switchover to a high-pressure, lowvolume operation being controlled by the action of the pilot-operated unloading valve 57. The changeover pressure is determined by the pressure of the air supply as well as the ratio of the area of the pilot piston 71 to the area of the poppet valve member 72. When the pressure of the hydraulic fluid is sufficient to cause movement of the poppet valve member 72 against the action of the pilot piston 71, the unloading valve poppet member 72 moves to the position shown in FIG. 2 whereby fluid flow from the large-volume chamber flows past the poppet valve member 72 to a passage which connects to a passage 111 in the end plate 10 having a connecting tube 112 extending down into the reservoir. This releases the pressure in the largevolume chamber so that the air pressure acting against the face 21 of the air-operated piston 20 is now acting to move the piston 46 to pump high-pressure fluid from the chamber 45. This fluid acts against an end of the poppet valve member 72 to hold the unloading valve open. The hydraulic pressure continues to build until the pressure thereof acting against an end of the piston 46 is balanced by the force of the air acting against the piston face 21. The maximum pressure reached is controlled by controlling the air pressure provided to the system by the setting of the air pressure regulator 53.

The position of the booster assembly and control components at the time of shifting to the low-volume, high-pressure condition is shown in FIG. 2 wherein the flow from the large-volume chamber is through bore 36 and past the poppet valve member 72 of the unloading valve with flow then through passages 110 and 111 and tube 112 to the reservoir, as shown by the arrows. High pressure fluid from chamber 45 flows to the operating device 50 with the check valve 56 and the poppet valve member 67 of the release valve 55 being on their seats to prevent flow of high pressure fluid to the passages leading to reservoir. As seen in FIG. 2, continued upward movement of the air-operated piston 20 will cause the piston 46 to deliver fluid at a high pressure from the chamber 45 with the ratio between the area of the airoperated piston 20 and the piston 46 being relatively large to create the high pressure.

As stated above, the final pressure capable of being applied to the operating device 50 is controlled by the setting of the air pressure by the air pressure regulator 53.

The pilot-operated check valve 58 is provided as a safety control to maintain hydraulic pressure at the operating device 50 in the event that air pressure is lost in the control circuit. The pilot-operated check valve has abody 115 through which the passages 75 and 90 extend. These passages intersect a central bore. A poppet-type check valve 116 is normally urged against a seat 117 by a spring 118 to block hydraulic flow through the passage 90. The poppet valve member 116 has an internal seat which is normally closed by a small ball 120, urged toward its seat by the spring 118. An actuating plunger 12] for the small ball 120 is loosely mounted in a flow passage 122 in poppet valve member 116 and extends outwardly from an end thereof.

An air-operated piston 130 is also mounted in the bore of the body 115 and engages an actuating rod 131. The air-operated .piston and the actuating rod are both urged to the right, as viewed in FIG. I, by a spring 132.

In the first stage operation, shown in FIG. 1, and the second stage operation, shown in FIG. 2, hydraulic pressure maintains the poppet valve member 116 away from its seat 117 to permit hydraulic flow to the operating device 50. When the required pressure is reached and hydraulic flow stops, the valve member 116 and the small ball 120 are seated by the spring 118 and hydraulic pressure is maintained if there is a loss of pressure air. I

When it is desired to release the pressure on the operating device 50, the pilot valve 51 is moved to the position shown in FIG. 3 to exhaust pressure air from'the air control valve 52 through an exhaust port 220 whereby the spring 62a moves the valve spool 62 of the air control valve to its upper position, as viewed in the Figure. This connects passage 64 to the exhaust line 78 whereby pressure air previously acting on the righthand face of the control piston 71 of the unloading valve and on the left-hand face of the control piston 65 of the release valve 55 is released. Passage 80 at the upstream side of the quick exhaust valve 83 is also connected to exhaust through the passage 64. This permits air under pressure beneath the air-operated piston 20 to quickly pass to exhaust 78 by upward movement through the tube 22 and to the right of the quick exhaust valve, as shown in FIG. 3. Release of the air pressure from the underside of air-operated piston 20 also releases the hydraulic pressure, since there is no longer any force acting against the high pressure piston 46.

Pressure air from the air supply 60 passes through the air control valve 52 to the line 76 and through line 75 to move the piston 91 of the operating device 50 downwardly. If the operating devices 50 have spring return cylinders, then the port in check valve 58 from which passage 75 extends can be capped and passage 75 not used. Pressure air in passage 76 is also directed to the right-hand face of the control piston 65 of the release valve by communication therewith through a passage 225 which shifts the control piston and the stem 66 thereof toward the left, as viewed in FIG. 3, to move the poppet valve member 67 off its seat to open the valve and permit return flow of hydraulic fluid to the reservoir by communication with passage 110.

Return flow of hydraulic fluid to the reservoir additionally requires opening of the pilot-operated check valve 58. This is accomplished by the pressure air in passage extending through'the pilot-operated check valve 58. This pressure air acts on the righthand end of the air-operated piston 130 to move the actuating rod 131 to the left to engage and move the plunger 121 to unseat the small ball 120. Hydraulic flow from the operating device 50 can flow to the left-hand end of poppet check valve 116 through an annular passage be tween the outer diameter of the poppet check valve 116 and the bore in the valve body 115. The fluid then flows through the low passage 122 in the valve member 116.

When the pressure of the hydraulic fluid decreases by flow through flow passage 122 to the point where the force exerted by the air-operated piston 130 exceeds the force of the hydraulic pressure against the left-hand end of the poppet check valve 116, the latter valve is lifted from its seat, allowing free flow of hydraulic fluid from the operating device 50 and as shown in FIG. 3.

Pressure air in passage 76 is also supplied to passage to direct pressure air into the chamber 40 where it acts upon the upper exposed end of the annular member 30 to force the air-operated piston 20 downwardly along with the parts connected thereto. During this downward movement to the initial position of FIG. 1, hydraulic fluid flows from the operating device 50 past the open poppet valve member 67 of the release valve, as previously described, and also directly to chamber 45 through the passage 92. The return flow past the open poppet valve member 67 may flow to the lower pressure chamber through passage 36 by flow to passage 95 and past the poppet valve member 72 of the unloading valve which is off its seat, as shown in FIG. 3. The large-volume chamber may also receive flow through the passage by movement of a check valve 230 off its seat to connect passage 110 with a space 231 at the end of the block 32 which opens to the passage 36. At the same time, hydraulic fluid, which was unloaded to the reservoir during the high pressure stroke, is returned to the large-volume chamber by being drawn past the check valve 230.

With the structure disclosed herein, it is possible to have a two-stage air-hydraulic booster with a control providing for an adjustable setting of the ultimate pressure and which automatically controls the setting at which the shift from low to high pressure will occur. Additionally, a minimum amount of air is used in operating the airoperated piston of the booster. The minimal use of air in return of the air-operated piston to initial position results in faster action.

We claim:

1. A two-stage air-hydraulic booster having a cylinder with a movable piston therein, means including a first member on said piston defining a large-volume low-pressure hydraulic chamber, means including a second member on said piston defining a small-volume high-pressure hydraulic chamber, air passage means for selectively directing air against a face of said movable piston to cause movement thereof from an initial position and force hydraulic fluid from said chambers, and

second air passage means for selectively directing air against one of said members to return said movable piston to said initial position.

2. A booster as defined in claim 1 wherein said first member is an annular sleeve extending from a face of the piston opposite said first mentioned face thereof, and said air for returning the piston to said initial position is directed against the exposed end of said annular sleeve.

3. A booster as defined in claim 2 wherein said means defining the small-volume chamber includes a block with a bore receiving the second member on the piston, said annular member being fitted onto said block to define said large-volume chamber therebetween, and a second bore in said block defining a hydraulic flow passage to said large-volume chamber.

4. A booster as defined in claim 3 including a hydraulic fluid reservoir, said reservoir being annular and having an inner wall surrounding said annular sleeve whereby said inner wall and said block define a chamber to receive said annular sleeve and the return air for returning the piston to said initial position.

5. A booster as defined in claim 1 including a control circuit with means for selectively directing air under pressure to either said face of the piston or against said first member, and said second air passage means including a quick exhaust valve responsive to air pressure caused by movement of said piston toward said initial position to connect said second air passage means to exhaust,

6. A booster as defined in claim 5 wherein said quick exhaust valve is responsive to the supply of air under pressure to said piston face to block said connection to exhaust.

7. A booster as defined in claim 5 wherein said'control circuit includes a shiftable valve for selectively connecting said large-volume chamber to a hydraulic reservoir, and an air operated control piston associated with said valve whereby said valve shifts when the force thereagainst exceeds the force against said air operated control piston, and an air pressure regulator for controlling the pressure of the air acting against said air operated control piston.

8. A two-stage air-hydraulic booster having a body with a cylinder with a movable piston therein, a cylindrical block extending from one end of the body toward said piston, an annular housing fitted to said end of the body to define a hydraulic reservoir and with the inner wall of said housing being spaced from the cylindrical block, an annular sleeve extending from one face of said piston and into the space between said block and said annular housing whereby a large-volume low-pressure chamber is defined within said sleeve, a first bore in said block and a plunger in said bore and extending from said one face of said movable piston whereby a small-volume high-pressure chamber is defined in said first bore, means for selectively directing air under pressure to said cylinder and against a face of said piston opposite said one face to move said piston from an initial position and reduce the size of said chambers, and means for selectively directing air under pressure against an exposed end of said annular sleeve to return said movable piston to said initial position.

9. A booster as defined in claim 8 wherein said block has a second bore communicating with said largevolume low-pressure chamber.

10. A booster as defined in claim 8 including a hydraulic valve for connecting said large-volume chamber to either an outlet or reservoir, and means providing for shift of said valve to complete the connection to said reservoir when a predetermined pressure exists in said largevolume chamber.

11. A booster as defined in claim 10 including a control circuit having air and hydraulic components including said hydraulic valve, and said shifting means includes an air-operated control piston which acts to yieldably hold said valve against shifting.

12. A booster as defined in claim 11 including a pilotoperated check valve for preventing return flow to the reservoir upon loss of pressure air in said control circuit.

13. A booster as defined in claim 8 including an air exhaust passage connected to a passage leading to said cylinder and a quick exhaust valve positioned at the entrance to said exhaust passage and responsive to air under pressure being supplied to said cylinder to close the exhaust passage and responsive to reverse air flow as said movable piston returns to initial position to open said exhaust passage for quick return of said movable piston.

14. A booster as defined in claim 8 including a control circuit with air-operated components, a device operated by hydraulic fluid supplied from said chambers, and means for preventing return of hydraulic fluid to said chambers upon loss of pressure air in the control circuit.

15. A two-stage booster having a body with a cylinder with a movable piston therein, a cylindrical block extending from one end of the body toward said piston, an annular housing fitted to said end of the body to define a hydraulic reservoir and with the inner wall of said housing being spaced from the cylindrical block, an annular sleeve extending from one face of said piston and into the space between said block and said annular housing whereby a large-volume low-pressure chamber is defined within said sleeve, a first bore in said block and a plunger in said bore and extending from said one face of said movable piston whereby a small-volume high-pressure chamber is defined in said first bore, means for selectively directing control fluid under pressure to said cylinder and against a face of said piston opposite said one face to move said piston from an initial position and reduce the size of said chambers, and means for selectively directing control fluid under pressure against an exposed end of said annular sleeve to return said movable piston to said initial position. 

1. A two-stage air-hydraulic booster having a cylinder with a movable piston therein, means including a first member on said piston defining a large-volume low-pressure hydraulic chamber, means including a second member on said piston defining a smallvolume high-pressure hydraulic chamber, air passage means for selectively directing air against a face of said movable piston to cause movement thereof from an initial position and force hydraulic fluid from said chambers, and second air passage means for selectively directing air against one of said members to return said movable piston to said initial position.
 2. A booster as defined in claim 1 wherein said first member is an annular sleeve extending from a face of the piston opposite said first mentioned face thereof, and said air for returning the piston to said initial position is directed against the exposed end of said annular sleeve.
 3. A booster as defined in claim 2 wherein said means defining the small-volume chamber includes a block with a bore receiving the second member on the piston, said annular member being fitted onto said block to define said large-volume chamber therebetween, and a second bore in said block defining a hydraulic flow passage to said large-volume chamber.
 4. A booster as defined in claim 3 including a hydraulic fluid reservoir, said reservoir being annular and having an inner wall surrounding said annular sleeve whereby said inner wall and said block define a chamber to receive said annular sleeve and the return air for returning the piston to said initial position.
 5. A booster as defined in claim 1 including a control circuit with means for selectively directing air under pressure to either said face of the piston or against said first member, and said second air passage means including a quick exhaust valve responsive to air pressure caused by movement of said piston toward said initial position to connect said second air passage means to exhaust.
 6. A booster as defined in claim 5 wherein said quick exhaust valve is responsive to the supply of air under pressure to said piston face to block said connection to exhaust.
 7. A booster as defined in claim 5 wherein said control circuit includes a shiftable valve for selectively connecting said large-volume chamber to a hydraulic reservoir, and an air operated control piston associated with said valve whereby said valve shifts when the force thereagainst exceeds the force against said air operated control piston, and an air pressure regulator for controlling the pressure of the air acting against said air operated control Piston.
 8. A two-stage air-hydraulic booster having a body with a cylinder with a movable piston therein, a cylindrical block extending from one end of the body toward said piston, an annular housing fitted to said end of the body to define a hydraulic reservoir and with the inner wall of said housing being spaced from the cylindrical block, an annular sleeve extending from one face of said piston and into the space between said block and said annular housing whereby a large-volume low-pressure chamber is defined within said sleeve, a first bore in said block and a plunger in said bore and extending from said one face of said movable piston whereby a small-volume high-pressure chamber is defined in said first bore, means for selectively directing air under pressure to said cylinder and against a face of said piston opposite said one face to move said piston from an initial position and reduce the size of said chambers, and means for selectively directing air under pressure against an exposed end of said annular sleeve to return said movable piston to said initial position.
 9. A booster as defined in claim 8 wherein said block has a second bore communicating with said large-volume low-pressure chamber.
 10. A booster as defined in claim 8 including a hydraulic valve for connecting said large-volume chamber to either an outlet or reservoir, and means providing for shift of said valve to complete the connection to said reservoir when a predetermined pressure exists in said large-volume chamber.
 11. A booster as defined in claim 10 including a control circuit having air and hydraulic components including said hydraulic valve, and said shifting means includes an air-operated control piston which acts to yieldably hold said valve against shifting.
 12. A booster as defined in claim 11 including a pilot-operated check valve for preventing return flow to the reservoir upon loss of pressure air in said control circuit.
 13. A booster as defined in claim 8 including an air exhaust passage connected to a passage leading to said cylinder and a quick exhaust valve positioned at the entrance to said exhaust passage and responsive to air under pressure being supplied to said cylinder to close the exhaust passage and responsive to reverse air flow as said movable piston returns to initial position to open said exhaust passage for quick return of said movable piston.
 14. A booster as defined in claim 8 including a control circuit with air-operated components, a device operated by hydraulic fluid supplied from said chambers, and means for preventing return of hydraulic fluid to said chambers upon loss of pressure air in the control circuit.
 15. A two-stage booster having a body with a cylinder with a movable piston therein, a cylindrical block extending from one end of the body toward said piston, an annular housing fitted to said end of the body to define a hydraulic reservoir and with the inner wall of said housing being spaced from the cylindrical block, an annular sleeve extending from one face of said piston and into the space between said block and said annular housing whereby a large-volume low-pressure chamber is defined within said sleeve, a first bore in said block and a plunger in said bore and extending from said one face of said movable piston whereby a small-volume high-pressure chamber is defined in said first bore, means for selectively directing control fluid under pressure to said cylinder and against a face of said piston opposite said one face to move said piston from an initial position and reduce the size of said chambers, and means for selectively directing control fluid under pressure against an exposed end of said annular sleeve to return said movable piston to said initial position. 